Several micro-display projection (MDP) technologies are currently available in the market place targeting 40″ to 70″ TV screen diagonal sizes. For example, digital light processor (DLP) based projectors incorporate binary intensity modulation at the pixel level and typically rely on a single panel to temporally multiplex (in a time-sequential manner) red, green and blue (RGB) color channel information of an image. On the other hand, both transmissive liquid crystal display (xLCD) and liquid crystal on silicon (LCOS) projectors utilize the electro-optic effect of a switchable LC layer to provide pixel-level modulation. Since the fabrication of polarization-based xLCD and LCoS MDP panels are typically lower cost and higher yield than the fabrication of millions of hinged micro-mirrors on a DLP backplane, both XLCD and LCoS optical engines are often configured with a three-panel architecture, where the RGB color channels are simultaneously displayed and converged before being projected to a screen. While LCoS panels may be based on either twisted-nematic (TN) or vertical-aligned nematic (VAN) liquid crystal (LC) layers, VAN-mode LC technology is generally more prevalent in commercial LCoS based projectors. Although the industry is transitioning to VAN-mode LC in xLCD panels, the prevalent LC mode of operation in xLCD panels is TN.
Optical engines using three TN xLCD panels have been promoted under the “3LCD” industry forum. A sub-system of the 3LCD architecture is schematically illustrated in FIG. 1, which shows an image modulation segment of a typical 3-panel light engine. The optical sub-system 100 includes input pre-polarizers 101a, 101b, 101c, retarder compensators 103a, 103b, 103c, xLCD panels 104a, 104b, 104c, and exit clean-up polarizers 105a, 105b, 105c. The center element of the optical sub-system 100 is an X-cube 110, where three separate light beams 120a, 120b, 120c are aggregated and emitted as a converged light beam 130, which is projected onto a screen (not shown). The three separate light beams provide the RGB channel data. In general, the green channel often corresponds to the first light beam 120a so that it is directed at the transmitted port of the X-cube. For each color channel, the xLCD panel 104a/104b/104c is positioned between a set of crossed polarizers (e.g., between an input pre-polarizer 101a/101b/101c and an exit clean-up polarizer 105a/105b/105c, respectively). In the schematic shown, the input pre-polarizers 101a, 101b, 101c have their transmission axes aligned horizontal (parallel to plane of drawing), while the exit clean-up polarizers 105a, 105b, 105c have their transmission axes aligned vertical. The arm of the optical sub-system 100 corresponding to the green or ‘a’ channel typically includes a half-waveplate (HWP) 106 to convert the modulated vertically polarized light to horizontally polarized light so that it appears as P-polarized light with respect to the X-cube hypotenuse and is transmitted through the X-cube. Alternatively, if the xLCD panel 104a rotates the incoming vertical polarization to horizontal polarization in the on-state, the HWP 106 may be positioned in another arm of the optical sub-system 100.
The retarder compensators 103a, 103b, 103c are compensating elements used to improve the contrast level of the xLCD MDP system, which is otherwise reduced when the panel is viewed obliquely. For example, it is well known that the refractive index anisotropy in TN-mode LCD panels degrades the viewing angle characteristic of the xLCD MDP system. In the absence of retarder compensators 103a, 103b, 103c, the xLCD native panel contrast is typically a few hundred to one. With the retarder compensators 103a, 103b, 103c, the compensated xLCD panel contrast is substantially higher.
Conventionally, the retarder compensators 103a, 103b, 103c have been fabricated out of stretched organic foil, such as Fuji's Wide View (WV) film, which consists of a discotic layer on a triacetate cellulose (TAC) substrate. The use of stretched organic foils as retarder compensators in MDP systems is likely rooted to the use of the same in the direct view LCD industry, where large screen areas (e.g., 2.5 inches or larger) need to be compensated for contrast and/or to improve viewing angle. However, in MDP applications, the increased light flux may result in premature degradation of these organic retarder compensators. In addition, the uniformity and surface quality specifications required for small screen areas (e.g., 2.5 inches or smaller) is not always met with these organic retarder compensators. Accordingly, a more reliable retarder technology as a contrast enhancement solution is desired.
One such solution was proposed in US Pat. Appl. No. 20060268207, the entire contents of which are hereby incorporated by reference. In this reference, Tan et al disclose using a tilted C-plate retarder as a contrast enhancer in both transmissive (e.g., XLCD) and reflective (e.g., LCoS) MDP systems. The tilted C-plate retarder is fabricated with vacuum coated dielectric layers, and thus exhibits high reliability and high retardance uniformity. Notably, using vacuum coated dielectric layers to form a C-plate element is also described in U.S. Pat. No. 7,170,574, with is also hereby incorporated by reference.
Referring to FIG. 2, the optics of one arm of the prior art xLCD MDP system using a tilted C-plate retarder compensator is shown. In this sub-system 200, a cone of light output from a prior stage light pipe (or other homogenizer such as Fly's Eye Array, not shown), is linearly polarized by the pre-polarizer 201. The transmission axis 220 of the pre-polarizer 201, which can be aligned arbitrarily over the entire circle, is typically aligned at ±45′, 0° or 90° with respect to the x-axis (shown aligned at 0°). Light transmitted through the pre-polarizer 201 is passed through the retarder compensator 203 and the xLCD imager 204, the latter of which typically has its slow axis 230 aligned at ±45° azimuthal offset 235 versus the pre-polarizer transmission axis 220. Light passed through the xLCD imager 204 is then transmitted to a post-analyzer 205, which typically has its transmission axis 221 aligned perpendicular to the pre-polarizer axis 220.
While this optical system 200 is shown to include only one retarder compensator 203, which is disposed between the pre-polarizer 201 and the xLCD imager 204, alternate embodiments provide one or more stages of retarder compensator that may be inserted anywhere between the pre-polarizer 201 and the post-analyzer 205. For example, in another embodiment the retarder compensator 203 is disposed between the xLCD imager 204 and the post-analyzer 205. In yet another embodiment a first retarder compensator 203 is provided between the pre-polarizer 201 and the xLCD imager 204, while a second retarder compensator (not shown) is provided between the xLCD imager 204 and the post-analyzer 205.
In each case, the retarder compensator 203 includes a C-plate retarder mounted at an angle to the x-y plane. More specifically, the C-plate retarder 203 is tilted such that it is aligned at a polar angle tilt 211 with respect to the system x-axis and at a polar angle tilt 212 with respect to the system y-axis. This two-dimensional tilt sets the axis of rotation 240 at azimuthal angle 245 with respect to the x-axis. The axis of rotation 240 is parallel to the plane of the xLCD imager 204 and parallel to the system x-y-plane. The z-axis is the propagation axis of the principal ray, which is also referred to as the transmission axis.
The assignment of fast/slow axes of the tilted C-plate retarder 203 relative to the axis of rotation 240 is dependent on the sign of C-plate retardance. For a −C-plate, the slow axis (SA) lies on the tilted surface at azimuthal angle 245, which is nominally perpendicular to the imager SA 235. For a +C-plate, the fast axis (FA) lies on the tilted surface at azimuthal angle 245, which is nominally parallel to the imager SA 235. The terms “nominally perpendicular” and “nominally parallel” are used to reflect the common practice in retardation compensation of rotating or clocking the retarder compensator SA from perpendicular alignment relative to the imager SA 235 by small value.
Advantageously, the tilt of the −C-plate introduces a net retardance, as seen by the principal ray, having a magnitude that provides compensation for the residual in-plane retardance of the xLCD panel in the dark state. In addition, the form-birefringent coating on the tilted C-plate provides a retardance profile (with incident angle) that provides compensation for the residual out-of-plane retardance of the xLCD panel in the dark state. In other words, a single −C-plate-only component is used to provide both on-axis and off-axis retardance compensation for the xLCD MDP system, thus providing a high contrast image with minimal components.
While the tilted C-plate-only retarder compensator has shown potential for use in both LCoS and xLCD MDP systems, where its durability in high light flux environments and highly uniform retardance characteristics are advantageous, it is limited in that it does not allow for the decoupling of the fast/slow axes from the geometric tilt-plane. In fact, the tilted C-plate retarder is a geometric retarder, wherein the FA and SA are set by the plane of incidence (e.g., as discussed above, the SA plane in the tilted −C-plate is the tilt plane).
Since the SA and FA are set by the plane of incidence, it is more challenging to fabricate a geometric retarder having a linear retardance profile that matches the linear retardance requirements of a given panel (e.g., which may exhibit a characteristic asymmetry in its conoscopic linear retardance profile along one of the slow or fast axes).
It would be advantageous to provide a retarder compensator that provides similar durability and/or retardance uniformity characteristics provided by the tilted C-plate retarder compensator, wherein the FA and SA are not determined by the plane of incidence.